1. Field of the Invention
The invention relates to a method and apparatus for use in the field of oil and gas recovery. More particularly, the invention relates to wireless, e.g., acoustic, downhole detection, monitoring and/or communication.
2. Description of the Related Art
A common method of drilling or extending a wellbore uses a drill bit turned by a positive displacement motor (PDM), which is mounted at the lower extremity of a pipe. The pipe may be made up of discrete lengths joined together or may be a single continuous length. The motive power for the PDM is provided by pumping a fluid into the upper extremity of the pipe, at or above ground level.
The fluid driving the PDM may comprise one-phase fluid or two-phase fluid. A one-phase fluid is substantially liquid. A two-phase fluid contains a significant fraction of gas. The reason for choosing to pump one or two-phase fluids depends on the drilling conditions, but a chief reason for using two-phase is to ensure that the fluid pressure created in the wellbore will not cause damage to the rock formation.
Where the pipe is relatively small in volume and where the fluid is one-phase the operator of a pump usually will have no difficulty determining whether the PDM is turning at the intended rate because the rate can be inferred at the surface from the pump pressure and flow values. However, where the pipe is relatively large in volume and/or where the fluid is two-phase the operator may have difficulty in determining the operating status of the PDM. This is because the pressure response caused by a variation in turning rate of the PDM is dampened by the volume of the pipe and/or gas in the pipe.
The consequence of an inability to determine the operating status of the PDM is that corrective action may not be taken to avoid damage to the drill bit. A drill bit may stop turning due to excessive load (“stall”) or it may lose contact with the rock. The consequences of a stall are lack of drilling progress and potential damage to the PDM. The consequences of losing contact with the rock are lack of drilling progress and excessive speed, potentially leading to damage to the PDM.
Prior to this invention, operators used numerous methods to infer the status of a PDM, including detecting vibrations in a pipe using a downhole detection transducer and subsequently communicating information to the surface using a communications transducer. These prior art methods generally rely on relatively high frequency vibrations. It will be understood that the action of a drill bit causes the pipe to vibrate and, to some extent, these vibrations travel through the pipe. These prior methods include simple methods, such as placing the ear in contact with the pipe, and more sophisticated methods, such as employing a sensitive detector (e.g. microphone, accelerometer, geophone) to detect the vibration, amplifying the detected signal to audible levels, and feeding an audible signal to headphones or a loudspeaker for the benefit of the operator. Some sophisticated methods further include filtering, in an attempt to clarify the sound.
Additional problems with prior art methods include expense, reliability, and maintainability. In general, each additional downhole component introduces added development and product costs and insertion costs. Further, each component reduces overall reliability. Further still, maintenance and/or repair of failed downhole components are extremely expensive, if not impossible.
Much like downhole transducer vibration detectors, prior art acoustic downhole communication systems utilize relatively high frequencies. A disadvantage of such high frequency communications is that the signal strength rapidly diminishes as the wave propagates through the pipe. Such high frequency communications can be limited in use to a few thousand feet. In some cases, communications are restricted to periods of drilling inactivity.
There is a need for a reliable, maintainable, and cost effective downhole detection, monitoring and communication system. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.